High speed rotors and shafts, particularly those found in the hydrogen and oxygen turbopumps of the Space Shuttle Main Engines (SSME), are generally subject to vibrations and instabilities such as whirl instability, an orbital motion of high-speed shafts within bearings and seals, and which are generally due to tangential forces on the shaft. This whirling motion of a shaft is particularly damaging to bearings supporting the shaft, and necessitates use of damping seals and bearings that reduce or eliminate whirl instabilities and other undesirable vibrations. Another problem of high speed shafts is increased ball wear of ball bearings due to relatively high side loads applied to the shaft when in use, requiring expensive maintenance of the turbopumps. The damping bearings and gals also reduce this ball wear by reducing or eliminating these vibrations and size loads. These damping bearings and seals are developed and dynamically evaluated with a dynamic testing apparatus.
This testing apparatus uses a high speed steam turbine which spins a solid shaft supported by ball bearings, which shaft extending through the opening of the seal of bearing. A hydrostatic thrust bearing guides the test seal to prevent the seal from tilting, holding it in place such that a reciprocating shaker mechanism may vibrate the test seal. Shaker rods incorporating the strain gauges are tangentially attached to the seal bushing and to the shaker, and the shaker is operated in order to radially translate the seal or bearing. The resultant vibrations of the test seal or bearing are recorded and used to compute dynamic coefficients thereof.
this testing apparatus of the prior art is complex of construction, with the ball bearings of the high speed shaft being sensitive to any misalignment, necessitating expensive precision mounting in order to prolong life of the bearings. Additionally, the ball bearings are sensitive to transient interruptions of lubricant and coolant flow through the device. Also, connections to the shaker device are by mechanical linkages, including fork and clevis arrangements, which are subject to developing lash. Further, a number of seals are required to maintain separation between the high pressure test fluid, such as water, and lubrication oil and coolants. Additionally, the shaft of the tester, itself subject to axial loads and misalignment, configured such that pumps having a hollow shaft feed could not be simulated. Still further, the high pressures required for testing generated high axial thrusts on the test seal guide, causing rubbing. Also, the various fluids flowing through the tester and their attendant gals necessitated on longer shaft length, which in turn lowers the critical speed to approximately 10,000 rpm, at which whirl instabilities becomes a speed limiting factor of the test apparatus. Yet further, cryogenic fluids could not be tested because of the use of oil as a lubricant.
Accordingly, it is an object of this invention to provide a simpler and more reliable dynamic tester for damping seals and bearings wherein all rolling elements are eliminated and replaced by noncontacting, fluid bearings at each end of a hollow shaft and wherein axial thrusts are eliminated by use of a symmetrical reaction turbine to rotate the shaft. Additionally, the working fluid for the bearings is also used to drive the shaft via the turbine, allowing the shaft to be constructed shorter than the shaft of the device of the prior art and capable of much higher speeds well in excess of the aforementioned critical apes. Further, instrumentation is inserted and electrically connected to the tester from outside the tester, simplifying electrical connections and access to the instrumentation. Still further, a quadraplexed arrangement of the test seals allows tests with eccentric sleeves without overloading the fluid bearings. Also, the shaker rods are connected to the test seal and shaking device by flex hinges, which are not subject to developing lash.